This invention relates generally to inductive loop detectors which are suitable for vehicle detection. The principles of the invention are however applicable to any system in which the frequency of oscillation of an inductive loop is to be controlled.
Certain countries have stringent specifications which are applicable to inductive loop vehicle detectors. For example it may be specified that the loop excitation frequency must fall within an allocated band which extends from 62 to 78 kHz and during operation the loop frequency must not deviate from the specific authorized frequency by more than 0.1%.
Crystal oscillator excited loops have been used to obtain precise frequencies but these require switching of capacitor banks in order to trim the loop resonant frequency equal to that of the crystal oscillator. This is a cumbersome method which is usually manually implemented and is not suited to self-tuning designs. Another approach to the problem is shown in the specification of U.S. Pat. No. 4,459,561.